Within the automotive industry, High Intensity Discharge (HID) lamps are beginning to replace conventional incandescent halogen lights as lights for headlamps. In an HID lamp, light is generated by means of an electric discharge that takes place between two metal electrodes enclosed within a quartz envelope sealed at both ends. The main advantages of HID lamps are high lumen output, better efficacy and longer life. The HID headlamps available currently are Quartz Metal Halide lamps that are also used for general lighting.
The discharge medium in Quartz Metal Halide lamps consist of a mixture of xenon, mercury, sodium iodide (NaI) and/or scandium iodide (ScI3), wherein the surrounding envelope, or arc-tube, is made of quartz with tungsten electrodes protruding within the envelope. In operation, the lamp size is kept small enough for optical coupling purposes. Further, the lamps are required to meet the automotive industry standard of starting fast by delivering at least eighty percent of their steady state lumens no later than four seconds from the point at which they are turned on. The small lamp size and fast start requirements result in higher wall thermal loading, which in turn poses some limits on the quartz envelope material, and significant thermal stresses in the arc-tube, especially near the electrode roots. These limitations result in shortening the lamp life and also decreasing reliability of the lamp.
Because of improved reliability and performance, quartz in HID lamps is being replaced with ceramic material, such as polycrystalline alumina (PCA) and yttrium aluminum garnet (YAG). Ceramic arc-tubes can withstand higher temperatures and the cold spot temperature in ceramic lamps can be driven to a high enough value to evaporate the metal halide dose and produce enough vapor pressure for both the light emitting elements and the buffer gas. However, changing to ceramic material requires a change in the design of HID lamps to best optimize the thermal and structural integrity of the lamps.